use xplat nuget package for cake

Author: TGP

build.cake

@@ -1,6 +1,7 @@
 #tool "dotnet:?package=GitVersion.Tool&version=5.8.1"
 #tool "dotnet:?package=coveralls.net&version=3.0.0"
-#tool "nuget:?package=NuGet.CommandLine&version=6.0.0"
+#addin nuget:?package=NuGet.CommandLine.XPlat&version=6.0.0
+#tool nuget:?package=NuGet.CommandLine.XPlat&version=6.0.0
 #addin nuget:?package=Cake.Json&version=4.0.0
 #addin nuget:?package=Newtonsoft.Json
 #addin nuget:?package=System.Text.Encodings.Web&version=4.7.1